President Donald Trump walks with National Security Adviser H.R. McMaster from the Oval Office to Marine One on the South Lawn of the White House in Washington, Friday, June 16, 2017, for a short trip to Andrews Air Force Base, Md., then onto Miami. (AP Photo/Susan Walsh)

WASHINGTON (AP) — President Donald Trump confirmed Friday he is under federal investigation and appeared to single out a senior Justice Department official for criticism, firing off tweets underscoring his growing frustration with the persistent focus on Russia’s involvement in the 2016 election and possibly his campaign.

“I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt,” the president wrote.

The morning missive appeared to refer to Rod Rosenstein, the deputy attorney general. Last month, Rosenstein sent a memo to Trump raising concerns over FBI Director James Comey — concerns the White House then cited as a central reason for Comey’s firing.

Days after Comey was abruptly ousted, Rosenstein appointed Robert Mueller as special counsel to investigate Russian involvement in the 2016 presidential election and possible collusion with the Trump campaign. Mueller also has expansive powers to investigate any matters that develop from his initial investigation.

It was unclear whether the president’s comment confirming he was under investigation was based on direct knowledge or media reports that Mueller is examining whether the president obstructed justice by firing Comey. Still, the snowballing investigation has deeply angered Trump, who denies he has any nefarious ties to Russia. He’s increasingly focused his anger at both Rosenstein and Mueller, according to advisers and confidants, viewing the two as part of a biased effort to undermine his presidency.

Sen. Dianne Feinstein, the top Democrat on the Senate Judiciary Committee, said she was “increasingly concerned” that Trump will fire both Mueller and Rosenstein.

“The message the president is sending through his tweets is that he believes the rule of law doesn’t apply to him and that anyone who thinks otherwise will be fired,” Feinstein said. “That’s undemocratic on its face and a blatant violation of the president’s oath of office.”

Rosenstein has been overseeing the Russia probe since Attorney General Jeff Sessions recused himself. But Rosenstein, too, may ultimately have to hand off oversight of the probe given his own role in Comey’s firing.

Earlier this month, Rosenstein told The Associated Press that “if anything that I did winds up being relevant to his investigation then, as Director Mueller and I discussed, if there’s a need from me to recuse, I will.”

Trump’s tweets come after the top lawyer for his transition team warned the organization’s officials to preserve all records and other materials related to the Russia probe. An official of Trump’s transition confirmed the lawyer’s internal order, which was sent Thursday.

The transition official spoke on condition of anonymity because the person was not authorized to discuss post-election decisions publicly.

The order by the general counsel for the transition team casts a wide net on documents that could shed light on ties between Trump’s presidential campaign and representatives of Russia’s government. The order also covers separate inquiries into several key Trump associates including former National Security Adviser Michael Flynn, campaign adviser Paul Manafort, foreign policy aide Carter Page and outside adviser Roger Stone.

The White House has directed questions for details to outside legal counsel, which has not responded.

Vice President Mike Pence has also hired a private lawyer to represent his interests in the expanding probe. Pence headed the Trump transition until Inauguration Day.

The transition official said the organization has also separately asked the General Services Administration to preserve records from the Trump transition that were transferred to its facilities after the inauguration. The transition, a nonprofit structurally separate from the Trump campaign, continues to operate with a small staff.

The memo sent Thursday asks for records related to foreign travel, contacts with Russian “officials, agents or nationals” and background investigations into the top Trump associates now targeted by Mueller’s probe. The memo asks for preservation of electronic communications and data, telephone logs, audio recordings, videos, calendars and other items.

Friday’s tweets are the latest in a week of angry social media responses by the president after a report by The Washington Post that Mueller was looking into whether Trump obstructed justice.

“Why is that Hillary Clintons family and Dems dealings with Russia are not looked at, but my non-dealings are?” he asked at one point.

“They made up a phony collusion with the Russians story, found zero proof, so now they go for obstruction of justice on the phony story. Nice,” Trump wrote in his first tweet. “You are witnessing the single greatest WITCH HUNT in American political history – led by some very bad and conflicted people! #MAGA.”